284 R. G. Stress et al. 



of broods in Pond C, but only when the initial density was 10 Daphnia 

 liter ' . At the same time total biomass was 579 ^g C liter ' in the isolates 

 and only 267 /ug C liter ^ ' in Pond C. 



The upper limit of brood size for the first brood of D. 

 middendorffiana is roughly 10, much smaller than the limit of 20 or so for 

 D. pulex. Brood size in the natural populations with a mean of 4.5 was 

 therefore never at the reproductive limit. Although indirect, this is 

 evidence for food limitation in situ. Similar evidence comes from feeding 

 of fairyshrimps. Kangas (1972) showed that both species of fairyshrimps 

 were capable of a larger fecundity than was attained by the females in 

 Ponds Band C. 



Now, if the brood size is a valid indicator of food limitation, the mean 

 brood size of Daphnia shows that a limitation was present in early July 

 and the brood size of fairyshrimps shows that food limitation was also 

 present later in July when these animals produce their eggs. 



Another set of experiments in the tundra pondlets indicated that more 

 food was available in the pondlets than in the control ponds. In these 

 experiments, 5 Daphnia liter '^ were grown with either 0.5, 2, 5 or 10 

 fairyshrimp liter ~\ The results were similar to those from the previous 

 experiment in that brood size increased at first to 8.1 as the number of 

 fairyshrimps were increased but it then decreased to 4.3 at the maximum 

 density of fairyshrimps (Table 6-11). Daphnia biomass showed the same 

 pattern but the changes can be accounted for by the lower numbers of 

 surviving Daphnia at the high fairyshrimp densities. Again, the conclusion 

 must be that Pond C water in isolated pondlets can support a larger 

 biomass than is observed in the ponds. 



Model, observation, and experiment all show that reproductive effort 

 in Pond C may be food-limited. Production rate for the principal 

 generation is therefore food-limited. The "nested" pattern begins to 

 emerge. First of all, annual production in a body of water that is frozen for 

 9 months each year must indeed be temperature-limited. However, the 

 populations that are responsible for the synthesis of tissues have 

 apparently undergone an adjustment. Their life cycles are restricted to 

 essentially a single generation. Thus the number of generations, not 

 temperature, exerts direct control on annual productivity. Restriction to a 

 single generation is probably controlled intrinsically in the fairyshrimps 

 and is controlled environmentally in the Daphnia populations. In the 

 latter, temperature is involved but only as a cue in converting photoperiod 

 to "short day." Temperature therefore presents no direct limit to the 

 duration of the productive period. Indeed it is possible to show that extra 

 degree-days are available (Stross and Kangas 1969). 



Food limitation acts to control productivity, but within the principal 

 generation only. The evidence is that brood size is limited by food 

 concentration. There may be a compensatory system indicated in the 

 population, however. In simulation, a given number of young is more 

 efficiently produced by a part of a food-limited population. 



